It has been recognized for many years that light emitting structures are capable of providing a viable alternative for classic light sources. More particularly, LED devices have been fabricated which have reached high light output over an extended period of time.
LED devices typically include a layer of electroluminescent materials such as Aluminium gallium arsenide, Gallium arsenide phosphide, Aluminium gallium indium phosphide or the like which are generally described as direct band gap semiconductors. By employing quantum dot structures, power output of these devices could be enlarged.
While these devices rely on dedicated manufacturing technologies including various steps of epitaxial processing, other attempts have been made to integrate light emitting structures within commercially available technologies, which include CMOS fabrication processes as an example.
The vast majority of micro-electronic devices are formed in silicon. Over the last several decades, a substantial effort has been directed to refining the reliability and manufacturability of these devices. As a result, silicon-based microelectronic devices have become dependable and inexpensive commodity items. Particularly, Complementary Metal Oxide Semiconductor (CMOS) technology has become a multi-billion industry providing the basis manufacturing technology for nearly 90% of all electronic commodities to society. Furthermore, Silicon-on-Insulator (SOI) technology is regarded as a future basis technology for combining optoelectronics technology with mainstream electronics manufacturing technology.
To our knowledge, the current state of the art focuses on 1100 nm and above optical communication systems for application in CMOS and SOI, mainly as a result of compatibility with long haul optical fibre communication networks. This approach has serious limitation since it requires the incorporation of Ge in the systems in order to realize efficient detectors, and or 1110-V technology using hybrid approaches in both material and processing procedures. These technologies are extremely complex and also very expensive.
To take advantage of the existing silicon-based knowledge and infrastructure, there is a great interest in integrating active optical components into CMOS and SOI silicon technologies. Silicon, however, is an indirect band gap semiconductor material which, unlike a direct band gap semiconductor material it has low photon emission efficiency.
One source of visible light from silicon is a reverse biased p-n junction under avalanche breakdown conditions. Avalanche breakdown occurs when the p-n junction is reverse biased to the point of where the electric field across the junction accelerates electrons such that they have ionizing collisions with the lattice. The ionizing collisions generate additional electrons which, along with the original electrons, are accelerated into having additional ionizing collisions. As this process continues, the number of electrons increases dramatically, producing a current multiplication effect. A small percentage of these collisions results in photonic emissions through intra-band carrier relaxation effects, and inter-band carrier recombination effects.
Building on this principle, Snyman, et al. in an article “A Dependency of Quantum Efficiency of Silicon CMOS n pp LEDs on Current Density, IEEE Photonics Technology Letters, Vol. 17, No. 10, October 2005, pp 2041-2043” [12], have reported that the efficiency of light emission from silicon in such avalanching Silicon Light Emitting Device (Av Si LED) can be substantially increased by utilizing a reverse biased p-n junction with a wedge-shaped tip that confines the vertical and lateral electric field.
One of these attempts has been disclosed in U.S. Pat. No. 5,994,720. According to this disclosure an optoelectronic device is formed in a chip of an indirect bandgap semiconductor material such as silicon. The device comprises a visibly exposed highly doped n+ region embedded at the surface of an oppositely doped epitaxial layer, to form a first junction region closed to the surface of the epitaxial layer. When the junction region is reverse biased to beyond avalanche breakdown, the device acts as a light emitting device to the external environment. The device may further include a further junction region for generating or providing additional carriers in the first junction region, thereby to improve the performance of the device. This further junction can be multiplied to facilitate multi-input signal processing functions where the light emission from the first junction is a function of the electrical signals applied to the further junctions.
Although the optoelectronic device can be formed in a chip of an indirect bandgap semiconductor material, further improvements of the light output power are generally desirable in order to explore new applications of these devices, Also, it has recently been discovered that silicon nitride and silicon oxi-nitride materials as well as certain polymers offer very low loss optical waveguides at the longer wavelengths in the 650-850 nm regime.
Accordingly, what is therefore required is a light emitting structure which not only offers an easier integration into an existing commercially available manufacturing technology, but also one which provides an improved light output at the longer wavelengths, but below the Si detector threshold wavelength of 850 nm. On the other hand, generation at specific wavelengths and tuning of such wavelengths would also be most beneficial.